Commonly radar systems are active with a dedicated radio transmitter and a dedicated receiver. Passive radar systems do not have dedicated transmitters but use electromagnetic illumination available from external sources. A bistatic radar uses a single spatially separated transmitter and a single receiver. A multistatic configuration includes multiple transmitters and receivers. In passive radar, a signal reflected or scattered from an object is correlated with the signal originating at the transmitter as a reference signal.
Passive radar has several benefits including low costs of operation and maintenance, and an ability to operate covertly and non-intrusively. Hence, passive radar may be deployed in areas where normal radars cannot be deployed. Since a dedicated transmitter is not used, the passive radar system is of lower cost, physically small, and frequency allocation/licensing is not required. Disadvantages of passive radar include a reliance on third-party transmitters, the operator having no control over the availability of the illuminator, and low effective radiated power. Line of sight is required between the transmitter and the target, and between the target and the receiver. Line of site is further required between the receiver and the transmitter or another connection, e.g network connection, is required. Passive radar systems can be ground-based and fixed, or deployed on mobile platforms including submarines, ships and aircraft. Passive radars have been used or considered with transmitter illumination from terrestrial TV broadcasts, FM radio, cellular broadcasts, enemy radar systems and space platforms particularly communications and navigation satellite signals and global positional system satellite signals.
In a paper entitled “Bistatic radar using satellite-borne illuminators of opportunity”, Griffiths, H. D. et al. (International Conference: Radar 92) investigated the possibilities of using analog modulated television transmissions for bistatic radar.
Digitally modulated signals, particularly wide-band digitally modulated signals are commonly transmitted by geo-stationary earth orbit (GEO) satellites for wide-band communications and television broadcasts. A considerable number of satellites transmitting wide-band digitally modulated signals simultaneously illuminate a major part of the Earth's surface.
There is thus a need for, and it would be highly advantageous to have a method of system and method utilizing wide-band digitally modulated signals broadcast from GEO satellites for passive radar and motion detection.
Reference: http://en.wikipedia.org/wiki/Passive_radar
Background Noise of an Antenna
An antenna that looks toward a given background collects radiation according to Planck's blackbody radiation law. At frequencies that are lower then the maximal value of Planck's law, the mean power density Bf is calculated by the Rayleigh-Jones approximation to Planck's law:
            B      f        =                            2          *          k          *                      T            B                                    λ          2                    ⁢              dbm                              sq            .                                                  ⁢            meter                    *          Hz          *          str                      ,where:
k=Boltzman Cons tan t
λ=Wavelength
TB=Averaged Background Temperature
For down-looking antennas the radiation source is the terrain surface and for up looking antennas the radiation source is the sky. Integrating Bf over the upper hemisphere results in:
      B    e    =                    4        *        π        *        k        *                  T          B                            λ        2              ⁢                  ⁢          dbm                        sq          .                                          ⁢          meter                *        Hz            Be is known as the “spectral flux density”. Note that Bf and Be are both frequency dependent (non-white). When an antenna with area SA looks down from distance R, the approximate area that contributes to the noise is:
      A    =                            λ          2                *                  R          2                            S        A              ;Since the spatial angle is
  ω  =            S      A              R      2      STR.
The total amount of power that is collected by the antenna is:
            P      A        =                                        B            e                    *          A          *          ω          *                      W            S                                    4          *          π                    =              k        *                  T          B                *                  W          s                      ;where Ws is the band-width.Note that PA is frequency independent (it is white!). This fact enables the description of the antenna as a resistor with noise figure of 1 at temperature TB.In a more rigorous manner the antenna radiometric temperature is derived from the integration over the distribution of the background temperature in the field of view of the antenna. In order to accurately calculate the radiometric temperature, the temperature and the emissivity of the background must be known. 290K is considered the upper bound for Te, the earth background temperature. The precise value of the earth background temperature is a complex function of the type of the terrain, the weather and the direction of the antenna. Sky temperatures are usually lower then 100 K.
Diversity
Multiple pictures of the same object can serve as a tool to improve image quality. A certain object that is imaged more than once will result in certain differences or diversities between the pictures. Such differences are related to the viewing process and/or to the object itself. The sources for differences in the viewing process include: temporal, wavelength, polarization, direction of the source of the illumination of the object, direction and/or angle of viewer, noise and interference content. Typical differences within objects are originated by: location due to some kind of movement; or change in temperature.
Different elements of corresponding diversity images are either identical or different. The well known classification of all the elements of the different images includes: a “sum group” that contains all the elements of all the pictures, or the union between the different pictures. Identical elements i.e. the group of all the elements that exists in all the pictures individually, is known as “intersection group”. The third group is the difference group, given by the union group minus the intersection group.
Identical elements of multiple images can be used to improve detection when combined either in coherent or in non coherent manners. Differences between multiple images carry additional information that can improve the final processed image, however proper processing of union, intersection and difference must be performed. Union processing is used to reduce randomness (noise) at the picture it is based on different summation rules for the wanted and the noise elements, another application is the inclusion of missing elements into more complete pictures by various diversities (polarization, wavelength, directions etc.) Difference processing is the most common, on a ground fixed platform, a ground moving platform, a marine fixed platform, a marine moving platform, an airborne platform, or a space-borne platform. Preferably, data based on the received signal is dispatched to a second platform and a function is performed at the second platform, such as further processing and storing. Preferably, multiple space-borne transmitters emit multiple wide-band digitally modulated signals respectively and further processing includes three dimensional imaging and/or diversity imaging. Preferably, the coherent processing further within the viewed area moving target indication, moving target detection, mapping the viewed area, location determination of the target, tracking the target within said viewed area, classifying the target within the viewed area and identifying the target within the viewed area. Preferably, when multiple receiver antennas receive respectively multiple received signals three dimensional image processing and/or diversity image processing is performed based on the received signals
According to the present invention there is provided a radar image processed according to the methods as disclosed herein.
According to the present invention there is provided a passive radar system including a space-borne transmitter which broadcasts a wide-band digitally modulated signal over a region and illuminates the region; a receiver antenna oriented to receive radiation from at least one portion of the region. The portion is an area viewed by the receiver antenna, and a reference antenna oriented toward the space-borne transmitter. The reference antenna receives a portion of the wide-band digitally modulated signal. A coherent processing time duration is selected based on: (i) a radar cross-section of a target within the viewed area, (ii) a bandwidth of the wide-band digitally modulated signal, and iii) an antenna viewing angle of the receiver antenna. The antenna viewing angle is equal to a dimension of the receiver antenna divided by a range to the target. The received signal from the receiver antenna is coherently processed with a reference signal from the reference antenna, over a time interval greater than the coherent processing time duration. When the receiver antenna is traveling at a velocity over the region at a range from the target, the selection of coherent processing time duration is further based on said velocity and said range. The bandwidth is preferably greater than three hundred megaHertz or greater than four hundred megaHertz. Preferably, the bandwidth is a full bandwidth emitted by the spaceborne transmitter. Preferably, multiple space-borne transmitters emit multiple wide-band digitally modulated signals respectively and further processing includes three dimensional imaging and/or diversity imaging. Preferably, when multiple receiver antennas receive respectively multiple received signals, three dimensional image processing and/or diversity image processing is performed based on the received signals the differences between two adjacent aerial pictures is the source of the so called stereoscopic depth measurement. Other applications of difference processing is the detection of moving target (MTD) and identification of false targets. Intersection processing is used in order to compensate for various blur situations, a known use is tomography.
The term “wide-band” as used herein referring to a digitally modulated signal refers to an absolute value of bandwidth, (not relative to center frequency) typically greater than 300 MegaHertz (Mhz) A bandwidth of greater than 400 Mhz is used in different embodiments of the present invention. A preferred embodiment uses the entire frequency band, typically a broadband signal of 500 Mhz radiated by a satellite transponder, or multiple wide signals from multiple transponders of one or more satellites.
The term “dimension” as used herein referring to an antenna is typically a length, width or diameter of the antenna.
The term “transmitter” as used herein includes a transponder such as in satellite communications.